Titanium dioxide (TiO2) pigments are important pigments in the manufacture of paints, plastics, coatings and paper laminates. Titanium dioxide (TiO2) is produced commercially in two crystalline polymorphic forms, namely the rutile form which can be produced by both the chloride and sulfate processes and the anatase form which is produced by the sulfate process. Both of these processes are well known in the art. Generally, in the xe2x80x9cchloridexe2x80x9d process, rutile crystal is produced while in the sulfate process, both rutile and anatase crystals can be produced.
With regards to rutile TiO2 it is known to add volatile aluminum compounds in the oxidation reaction of titanium tetrachloride (TiCl4), primarily to increase the rutile content of the TiO2 pigment being produced. Typically, the amount of aluminum compound added is sufficient to form about 1 to 1.5 weight-percent alumina based on the TiO2 pigment, and this amount helps achieve greater than 99 weight-percent rutile in the pigment. The resulting TiO2 from the oxidation reaction is then subjected to certain finishing and grinding treatments depending on the desired type of final TiO2 pigment product.
Finishing and grinding treatment often involve the steps of: a) dispersing the TiO2 particles in an aqueous medium to form a slurry having a relatively low TiO2 solids concentration; b) precipitating hydrous inorganic oxide coating, such as for example, alumina, silica and/or other compounds, onto the surface of the TiO2 particles; c) recovering the inorganic oxide surface-treated TiO2 particles from the aqueous medium by filtering; d) washing the TiO2 particles to remove by-product salts and impurities; e) drying the washed TiO2 pigment particles; f) dry-grinding the dried TiO2 pigment particles to a desired particle size using a fluid-energy mill, and optionally, if an aqueous TiO2 pigment slurry is desired, g) dispersing the pigment product in water to produce a slurry typically having a TiO2 solids content of below about 78 weight-percent.
The titanium dioxide may be finished depending on the end-use application (i.e., paint, plastic etc). For example, some finishing steps involve wet surface treatments with other inorganic metal oxides, such as alumina, silica, phosphate, CeO2 and the like, for processibility and/or final applications as known in the art. The surface treatment steps then follow by drying and grinding. These steps will give the finished pigment the desired particle size, distribution and properties.
Generally, concentrated rutile TiO2 slurry products are made by dispersing the finished pigment product in water to produce a slurry having a TiO2 solids content below about 78 weight-percent. Slurries with solids contents greater than 78 weight-percent typically exhibit negative effects on stability and other physical properties (i.e. difficultly pumping, flocculation, gellation and poor or no fluidity over time).
Slurry stability may be further reduced when the rutile TiO2 is treated with an inorganic oxide such as amorphous alumina. Dispersants are often added to such slurries to make it maintain viscosity and storage stability so that it can easily be handled during transport from the TiO2 manufacturers to the customers. However, some prior art slurries, even with dispersants, tend to gel during conventional storage making them difficult to pump and transport.
Based on the foregoing, there is a need for titanium dioxide slurries having improved stability which are useful in the manufacture of paper laminates, plastic, paint and other coatings systems.
The present invention relates to photodurable slurries having improved stability that comprise: i) below about 78 weight-percent rutile titanium dioxide based on the total weight of the slurry, with a surface treatment comprising an amorphous alumina compound; ii) a polyacrylic acid dispersing agent having a weight average molecular weight (Mw) in the range of from about 2,000 to about 5,000, that is neutralized with a neutralizing agent having a monovalent group, and iii) water, wherein the slurry has a pH of from about 6 to about 8.
Compared to prior art slurries, the slurries of the present invention exhibit improved stability and a reduced tendency toward gellation over time. Accordingly, one advantage of the stable slurries of the present invention is that they can be easily handled and transported to other sites such as a paper laminate manufacturing facilities, while they maintain free flowing and pumpable characteristics.
In another embodiment, the present invention provides methods of making slurries having improved stability comprising mixing rutile titanium dioxide, surface-treated with an amorphous alumina compound, with water and a polyacrylic acid dispersing agent having a molecular weight in the range of from about 2,000 to about 5,000 under conditions so as to form the slurry, wherein the dispersing agent is neutralized with a neutralizing agent having a monovalent group, the mixing is performed at a pH of from about 6 to about 8 and wherein the rutile titanium is present in an amount below about 78 weight-percent based on the total weight of the slurry.
The improved stable slurries of the present invention enable many end-use improvements and expand the limits within which many end users, such as paper manufacturers, were heretofore required to operate.